Lithography is conventionally performed by a variety of systems and methods. Optical projection lithography employs a mask which is then imaged onto a substrate using either refractive or reflective optics, or a combination of the two. The mask contains the pattern to be created on the substrate, or a representation thereof.
FIG. 1 illustrates a cross sectional view of a conventional lithographic mask 10 separated from substrate 20 by a small gap, G. The mask 10 includes a support frame 11, membrane 12, and alignment marks 13. Complementary alignment marks 21 are located on the substrate and face the mask alignment marks 13.
FIG. 2 illustrates a prior art mask 10, showing four alignment marks 13. The central region of mask 10 includes pattern region 14 that contains the pattern that is to be superimposed over a pattern on the substrate.
FIG. 3 illustrates a substrate containing multiple identical regions of patterns 22 over which mask pattern 14 is to be superimposed in a sequence of three steps: (1) move to one of the multiple sites; (2) align mask alignment marks with respect to substrate alignments marks; (3) expose mask pattern 14 on top of substrate pattern 22.
Visible or near-infrared light is preferably used as the illuminating source for alignment, spanning a wavelength band from 400 to 900 nm, but other wavelengths may also be used. The light must be collimated with good spatial coherence. In addition, it is necessary that grating periods exceed wavelengths used in the illuminating bandwidth, so that first order diffraction is possible. The first order diffraction angle is sin θ=λ/g where θ is the diffraction angle, λ is the wavelength, and g is the period. Hence, first-order diffraction with λ/g>1 is not possible. It is desirable that grating periods exceed twice the value of any wavelengths used in the illuminating bandwidth, to ensure that first order diffraction occurs at an angle of no more than 30 degrees. Preferably, the alignment marks should include 10 or more periods of each of the paired gratings.
FIG. 4 illustrates a prior art arrangement of pairs of complementary alignment marks with differing periods to increase the range of freedom from measuring ambiguity. This feature increases the range of misalignment that can be measured without ambiguity, or increases the capture range.
In one arrangement the substrate alignment mark 21 includes two simple linear gratings having different spatial periods, gs1 and gs2. The mask alignment mark 13 includes two simple linear gratings having different spatial periods, gm1 and gm2. In between the gratings gs1 and gs2, or between the gratings gm1 and gm2 (latter is shown in FIG. 4) is an unpaired reference grating having a period gr which is coarser than any of gm1, gm2, gm1, and gm2. The periods gr, gm1, gm2, gm1, and gm2 are chosen so that:gr=gs1*gm1/|gs1−gm1|=gs2*gm2/|gs2−gm2|
This relationship ensures that gr is also the period of the two interference fringe patterns formed by the overlap of beams diffracted by the gratings. One such pattern results from interference of beams diffracted by gratings gs1 and gm1. The second such pattern results from interference of beams diffracted by gratings gs2 and gm2.
FIG. 5 illustrates the viewing of the pairs of facing alignment marks 21 and 13 by a microscope. Alignment occurs upon attainment of a predetermined phase difference. For example, this phase difference could be zero, in which case the microscope image would appear as in FIG. 6 when alignment occurs. When the relative mask and substrate positions are slowly changed, the fringe patterns will translate at higher rates. This translation not only results in magnification of the relative motion, but also overcomes the ambiguity problem that would arise with only one grating pair when the relative motion is a multiple of a grating period.
FIG. 6 illustrates the interference fringe patterns, and the reference pattern, observed when the alignment marks of FIG. 4 are properly superimposed. FIG. 7 illustrates an example of how the fringe patterns of FIG. 6 are shifted relative to each other when mask and substrate are relatively displaced perpendicular to the lines of the grating pairs.
In zone plate array lithography, an array of Fresnel zone plates is placed one focal distance away from the substrate. Each Fresnel zone plate can be individually addressed by a spatial light modulator to create an arbitrary dot-illumination matrix.
Further examples of systems using interference fringe patterns for interferometric detection of alignment and small gaps are described in U.S. Pat. No. 5,414,514; U.S. Pat. No. 6,088,103; and U.S. Pat. No. 6,522,411. The entire contents of U.S. Pat. No. 5,414,514; U.S. Pat. No. 6,088,103; and U.S. Pat. No. 6,522,411 are hereby incorporated by reference.
In the various lithographic methods discussed above, in which a pattern on a mask or template is transferred to a second planar surface, such as a silicon wafer or substrate, it is necessary to arrange the two surfaces so that the two surfaces are highly parallel, prior to bringing the two surfaces into close proximity or contact. A high degree of parallelism is important for control of consistent feature sizes and to avoid destructive contact between the template and substrate. In other applications, the surfaces may be required to be held at a specific angle or a specific large gap.
Conventionally, capacitive sensors have been used for gap detection in nano-positioning systems to detect gaps of <100 μm. Detectivity scales with the gap range—for a gap range of 10 μm, detectivity can be <1 nm. Moreover, conventionally, for a capacitive sensor designed for a gap range of 500 μm, detectivity is only ˜50 nm.
In a mask-substrate aligner, capacitive sensors are typically used to measure gap at three points around the periphery of the mask stage. Since the capacitive sensors measure the mask-substrate gap indirectly, calibration of the capacitive measurements to the actual mask-substrate gap is required. Errors associated with this methodology include thermal drift, variations between dimensions of individual masks, and stress-induced deformation by the mask stage.
Capacitive sensors have also been embedded within part of the substrate stage, adjacent to the substrate, and scanned underneath the mask. Some problems with this conventional embodiment are the potential mismatch between the capacitive sensing plane and the substrate plane, the time required to move the sensor and measure gap at multiple points, and perturbations in the capacitive measurements caused by patterned regions on the mask.
Aside from the various problems discussed above, the intrinsic limitations of capacitive sensors lie in the tradeoff between range and resolution because the capacitance falls off linearly with gap, G, between the plates, as illustrated by the following:
  C  =                    ɛ        r            ⁢              ɛ        o            ⁢              A        p              G  wherein εr is the relative permittivity of the material between the plates, εo is the permittivity of vacuum, and Ap is the area of the plates.
At large gaps, electronic noise dominates and limits gap range. At small gaps, noise is reduced, but sensitivity to non-parallelism of the capacitor plates is increased. In combination, these effects serve to limit the useful operating range.
In some applications, such as scanning probes, capacitive sensors are used to find the probe position in six axes, requiring six separate sensors. Since the range of capacitive sensors is very small compared to the diameter of the plates and the points of measurement are separated from each other and the scanning probe by a few centimeters, thermal expansion of the intervening lengths of material can degrade measurement accuracy.
Furthermore, in a multi-axis capacitor arrangement, the area of capacitor plate overlap will come into play during orthogonal motions; as one axis in an XY stage is moved, the area of plate overlap in the orthogonal axis will decrease, and may cause an undesirable variation in capacitance along the orthogonal axis. Measurement error at a point of interest between the two planar surfaces will result.
Another method of conventional gap sensing utilizes optical methods to provide a direct measurement between the mask and substrate. One such conventional method utilizes the interference of beams reflected from a HeNe laser focused onto the mask at an oblique angle. The diverging reflections from the mask and substrate interfered, and the interference fringes are detected by a linear photodetector array. The gap is determined by the spatial frequency of the interference pattern, which increases with gap linearly (to first order), with a small gap-dependent deviation from linearity. The range of gap detection is between 25 and 120 μm, and the gap uncertainty is a percentage of gap, ranging between 120 nm and 600 nm.
In proximity lithography, a typical exposure gap (i.e., <5 μm) is below the minimum detection range in the above-described optical method. At such small gaps, the angular disparity between reflected beams is too small to produce measurable interference fringes. Also, the finite thickness of the template causes multiple spurious internal reflections, which can corrupt the measurement signal.
It is further noted that the above-described conventional optical gap sensing method fails to integrate well with Interferometric Spatial-Phase Imaging aligning schemes, such as described in U.S. Pat. No. 5,414,514 and U.S. Pat. No. 6,088,103, since the above-described conventional optical gap sensing method requires diverging illumination, instead of the collimated illumination used in Interferometric Spatial-Phase Imaging. In addition, the geometry of the above-described conventional optical gap sensing method is incompatible with Interferometric Spatial-Phase Imaging alignment microscopes since the above-described conventional optical gap sensing method requires a different illumination angle and a separate sensor that must be on the opposite side of the mask from the illumination.
A second conventional optical gap sensing method measures a gap based on the interference of diffracted beams, using a beat signal that is dependent upon interference of a first-order beam and a second-order beam, diffracted from a grating. Each order has a different diffraction angle, which also depends upon the laser frequency: a laser beam of frequency f1 is incident at an angle θ1, and another beam with frequency f2 is incident at an angle θ3. The two orders are arranged to diffract at the same angle, θ2. The gap is linearly related to the phase of the beat signal. Gap sensitivity is ˜32 nm.
Although this second conventional optical gap sensing method can detect small gaps used in proximity exposures, the inseparability of gap and alignment signals is significant and undesirable.
Therefore, it is desirable to provide a method of directly measuring the gap. Moreover, it is desirable to provide an optical method of directly measuring the gap that does not require a different illumination angle and a separate sensor on the opposite side of the mask from the illumination. Also, it is desirable to provide an optical method of directly measuring the gap that can operate with collimated illumination. Furthermore, it is desirable to provide an optical method of directly measuring the gap that has separable gap and alignment signals.